Some of the country’s most significant sites have been caught in the crossfire in battles between regime forces and rebels. Others have been turned into military bases, raising archaeologists’ fears of damage.

The government’s shelling of neighborhoods where the opposition is holed up has smashed historic mosques, churches and markets. Looters have stolen artifacts from excavations and museums.

In one of the most egregious examples, shells thudded into the walls of the 12th century al-Madeeq Citadel, raising flames and columns of smoke as regime forces battled with rebels in March.

Local activists said regime forces carried out the assault and afterward moved tanks into the hilltop castle. Later Internet videos showed bulldozers knocking through part of the walls to create an entrance.

The government and opposition have traded blame for damaging and looting of sites around the country.

Blaming the government

A group of European and Syrian archaeologists tracking the threats through eyewitness reports from the ground blames the government. In several cases, Syrian troops have directly hit historic sites and looted them.

“We have facts showing that the government is acting directly against the country’s historical heritage,” said Rodrigo Martin, a Spanish archaeologist who has led past research missions inside Syria.

An important crossroads, Syria’s rich archaeological treasures extend over millennia.

The capital, Damascus, is often claimed to be the oldest continuously inhabited city in the world. Archaeologists have uncovered cities dating back 5,000 years to the early Bronze Age, and the country is dotted with hills that likely hide more such cities, still not excavated.

A series of cultures have left their mark - from biblical civilizations to Christian Crusaders and Muslim kingdoms.

“What we know of Syrian heritage has already provided a huge quantity of information, but we can safely say that the part that has not yet been studied is even bigger,” said Mr. Martin.

Each incident of destruction “is like burning a page in the book of history of mankind,” he said.

The heritage also helped fuel tourism, giving a much-needed economic boost before the uprising erupted more than a year ago. More than 8.5 million tourists visited Syria in 2010, 40 percent more than the year before. Now there are virtually none.

The 2,000-year-old Roman ruins of Palmyra - an ancient oasis city more than four centuries old and one of the biggest tourist draws - is deserted.

Government forces have surrounded the ruins and a nearby town and have set up a base in a historic castle on a hilltop overlooking the site, deep in Syria’s central deserts.

Heritage in the crossfire

Besides the break-in at Krak des Chevaliers in March, gunmen have also targeted a museum in the city of Hama. They have stolen antiques and a priceless gold statue dating back centuries, said Mr. Jammous, of the government’s museum agency.

Other sites have been endangered in the crossfire of the daily battles.

Several weeks ago, activists in the northwestern province of Idlib said, troops and dissidents battled near the ruins of Elba, a Bronze Age city where archaeologists in the 1960s discovered a massive trove of cuneiform tables that revolutionized their understanding of the ancient Mideast.

The once-bustling covered ancient market in old Homs - famous for its unusually tall arched roof where people bargained for colorful textiles, rugs, perfumes and clothes - has been heavily damaged. Its walls are now blackened from a fire. Its walkways littered with debris and shop shutters twisted and pierced with shrapnel.

Traditional Homs houses with arched doorways and inner courtyards have also been bombed.

Mosques have served as launching pads for anti-government protests in Syria. Government troops have targeted many of them, particularly in the provinces of Daraa, birthplace of the Syrian revolution.

Early on in the uprising, the government shelled Daraa’s Omari Mosque, built during the Islamic conquest of Syria about 1,400 years ago. Activists say government forces deliberately sabotaged the mosque and hid weapons inside it to prove that armed gangs were sheltering there.

Videos show the bombed-out minarets and shell-pocked facades of several mosques and churches in Homs. They include the Umm el-Zunnar church, which was built underground in 59 A.D.

In January, artillery fire struck the Sednaya Convent north of Damascus, believed to have been build in A.D. 547 A.D. The opposition blamed the attack on Syrian troops.

“They have absolutely no respect for the country’s cultural heritage,” said activist Tarek Badrakhan, speaking from Homs’ battered Khaldiyeh district. “Mosques, citadels, the old city, they spared nothing.”